Ministers yesterday defended the record of comprehensive schools at A-level as another round of record results revealed that the steady rise in A grades was largely fuelled by private and state selective schools.

The widening gap between state comprehensives and secondary moderns provided the most eye-catching statistics on a day when the traditional rows over whether the exams were being dumbed down were quieter than usual.

As the pass rate rose for the 25th consecutive year and more than 25% of exams were judged A grade for the first time, figures showed that improvements in recent years were unevenly spread between different types of school.

Exam boards said this proved standards had been maintained and that A-levels remained "incredibly highly valued and popular".

Mike Cresswell, director general of the largest exam board, AQA, said the numbers of A grades were rising far faster in independent and grammar schools. If exams were becoming easier, there would be improvements across all schools, he said.

"In terms of the proportion of people who get grade A, the gap between selective schools and independent schools on the one hand and other school types on the other hand has widened," he said.

Over the past five years the proportion of A grades awarded to pupils from independent schools has risen by 6.5%. The rate in comprehensive schools is 3%. Dr Cresswell said: "There would not be differences like this if the exams were getting easier.

Jim Knight, the schools minister in England, said that overall entries at A-level had risen by 8% in the last decade. He said: "More young people are now taking and achieving A-level qualifications which is something to celebrate." Sustained investment in mainstream education was the way to increase opportunity.

Michael Gove, spokesman for the Tories, said the results suggested a growing gap between the best-performing schools and the rest.

About 310,000 candidates entered more than 800,000 A-levels in England, Wales and Northern Ireland. Figures from the boards showed 25.3% of exams were graded A, up from 24.1% last year. In 1997 only 15.7% of all A-level scripts were A grades. Girls, as has become usual, scored better in every major subject apart from modern foreign languages and further maths, getting 26.5% of A grades against 23.9% for boys.

A-levels, last revamped in 2000, are about to be changed again, with students starting courses in September next year facing tougher, more stretching questions, extended projects and new A* grades which will be awarded from 2010. Dr Cresswell said the A* grade was an "eminently sensible" response to a problem of success. "You can see why a small number of universities have a problem differentiating between the very, very, very best and the very best."

In response to a question about whether there would ever be A**s, Dr Cresswell said: "Were one to find oneself in a situation at some point where things had improved to such an extent that there was now a similar difficulty with the A*, the sensible thing to do would be to repeat the medicine." But the A* would serve its purpose "for many years to come".

John Dunford, general secretary of the Association of School and College Leaders, was delighted by the revival in "hard subjects" such as maths, physical sciences and modern languages.

"Although these are recognised to be the hardest subjects, they are also the subjects in which the highest proportion get A and B grades because the brightest pupils take them."

His remarks were borne out by figures which showed nearly three in five entrants to further maths get A grades. Only 14% of entrants got grade A in media, film and TV studies.

Jonathan Shephard, of the Independent Schools Council, said: "We cannot afford either economically, morally or ethically the waste of talent that is occurring in this country. There is a wide consensus against selection, though not universal, but if all schools have to educate at least some children of the highest academic ability there are not enough suitably qualified teachers to go round.

"If the independent sector didn't exist whole university departments would crumble, particularly in the harder subjects, with a knock-on effect on their ability to stay at the top of world rankings."